The isolation and characteristics of six tremorgenic mycotoxins, penitrems A-F from cultures of Penicillium crustosum are reported. The assignment of structure (2) to penitrem A is based on a detailed study of its high-field 1H and 13C n.m.r. spectra. The conformation and relative configuration of penitrem A was deduced from the observed proton-proton nuclear Overhauser effects (n.O.e.) and the magnitude of the proton-proton coupling constants. The chirality of C-25 was determined as S by the' partial resolution' method of Horeau and penitrem A must therefore have the (12R, 14S, 15R, 18S, 19R, 22S, 23S, 24R, 25S, 26R, 28S, 31 R, 32S) absolute configuration.